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package org.apache.juneau.http;

/**
 * Represents a parsed <l>Expires</l> HTTP response header.
 * 
 * <p>
 * Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231).
 * 
 * <h5 class='figure'>Example</h5>
 * <p class='bcode'>
 * 	Expires: Thu, 01 Dec 1994 16:00:00 GMT
 * </p>
 * 
 * <h5 class='topic'>RFC2616 Specification</h5>
 * 
 * The Expires entity-header field gives the date/time after which the response is considered stale.
 * A stale cache entry may not normally be returned by a cache (either a proxy cache or a user agent cache) unless it is
 * first validated with the origin server
 * (or with an intermediate cache that has a fresh copy of the entity).
 * See section 13.2 for further discussion of the expiration model.
 * 
 * <p>
 * The presence of an Expires field does not imply that the original resource will change or cease to exist at, before,
 * or after that time.
 * 
 * <p>
 * The format is an absolute date and time as defined by HTTP-date in section 3.3.1; it MUST be in RFC 1123 date format:
 * 
 * <p class='bcode'>
 * 	Expires = "Expires" ":" HTTP-date
 * </p>
 * 
 * <p>
 * An example of its use is...
 * <p class='bcode'>
 * 	Expires: Thu, 01 Dec 1994 16:00:00 GMT
 * </p>
 * 
 * <p>
 * Note: if a response includes a Cache-Control field with the max-age directive (see section 14.9.3), that directive
 * overrides the Expires field.
 * 
 * <p>
 * HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past
 * (i.e., "already expired").
 * 
 * <p>
 * To mark a response as "already expired," an origin server sends an Expires date that is equal to the Date header
 * value.
 * (See the rules for expiration calculations in section 13.2.4.)
 * 
 * <p>
 * To mark a response as "never expires," an origin server sends an Expires date approximately one year from the time
 * the response is sent.
 * HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.
 * 
 * <p>
 * The presence of an Expires header field with a date value of some time in the future on a response that otherwise
 * would by default be non-cacheable indicates that the response is cacheable, unless indicated otherwise by a
 * Cache-Control header field (section 14.9).
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul class='doctree'>
 * 	<li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
 * </ul>
 */
public final class Expires extends HeaderDate {

	/**
	 * Returns a parsed <code>Expires</code> header.
	 * 
	 * @param value The <code>Expires</code> header string.
	 * @return The parsed <code>Expires</code> header, or <jk>null</jk> if the string was null.
	 */
	public static Expires forString(String value) {
		if (value == null)
			return null;
		return new Expires(value);
	}

	private Expires(String value) {
		super(value);
	}
}
